Start testing for battery voltage at the brake light switch, then FROM the closed (pedal pushed) brake light switch onward! Until you reach the point the voltage stops!!... Hint, if the signals work, forget about wiring and bulbs!!!!... Brake lights use that same circuit!!!.. .. .. Most of the time 'this' problem, as described, IS in the signal light (multifunction) switch.... Which can be tested also!!

In that case I suggest getting some hands-on help that is more familiar with this problem... It's just too much info needed to type it out!... Try YouTube... You 'might' find a helpful video.... Or for a fee you can go to www.alldatadiy.com